The present invention generally relates to dynamoelectric machines such as motors and generators. More particularly, the present invention relates to a permanent magnet brushless DC motor having an improved stator configuration to reduce cogging. Related U.S. Pat. Nos. 4,672,253 and 4,933,584 are incorporated herein by reference.
As is well known in the art, conventional permanent magnet brush-type DC motors are typically constructed having a stator and a rotor. The stator generally includes a number of permanent magnets housed within its core to provide the magnetic field for the rotor. The armature winding of the DC motor is typically placed in slots distributed across the face of an iron laminated rotor structure, wherein the armature coils are connected to the DC supply using a conventional brush/commutator arrangement. Although brush-commutated DC motors have several advantageous characteristics, including convenience of changing operational speeds, several disadvantages are also present. These disadvantages include brush and commutator heating, erosion of commutator and brush surfaces, electrical resistance, electrical noise, and radio frequency interference caused by sparking between the brushes and the segmented commutator. These disadvantages may limit the applicability of brush-type DC motors in some fields, such as the heating, ventilation, and air conditioning (HVAC) industry.
Most of the problems associated with the brush-commutated DC motor have been solved with the development of the brushless DC motor. The brushless DC motor is basically a permanent magnet DC motor, wherein the commutator and brushes of the conventional DC motor have been replaced by electronic switches to perform the commutation. With such an electronically commutated motor (ECM), the electronic circuitry switches the motor windings at the appropriate time to control the rotation of the machine. In the permanent magnet DC brushless motor, a permanent magnet rotor is housed within a stator core having an electronically commutated winding distributed among the stator teeth. Although various sensors and feedback electronics must be used in the ECM, most of the problems associated with the mechanical brush/commutator arrangement are avoided.
However, in a brushless DC motor having salient magnetic poles, a cogging torque occurs due to the salient pole structure. As is known to those skilled in the art, "cogging" is the non-uniform rotation of the rotor caused by the tendency of the rotor to prefer certain discrete angular positions. The degree of cogging is not only affected by the number of salient poles in the stator, but also by the structural configuration of the face of the salient pole. The cogging torque may be reduced by providing a substantially constant air gap energy. Placing a notch in the face of the salient pole essentially increases the air gap at that point, in order to impose a reluctance torque, caused by the stored energy in the notches, that is equal and opposite to the cogging torque. For example, in U.S. Pat. No. 4,672,253, the problem of cogging in a permanent magnet brushless motor is addressed by adding auxiliary grooves or notches on the face of the salient poles at precise locations determined as a function of the number of salient poles, auxiliary salient poles, and winding grooves. In U.S. Pat. No. 4,933,584, the cogging of an electronically commutated motor is substantially eliminated by skewing either the magnetic field of the rotatable assembly, or by skewing the notches in the face of the salient poles in the stator.
While the use of such notches may prove beneficial to reduce cogging, it has been found that the presence of notches has a detrimental effect on the demagnetization margin, as well as on the overall performance of the dynamoelectric machine. Demagnetization of the permanent magnets occurs to some extent whenever a current flows in the motor winding. The armature becomes, in effect, an electromagnet which tends to oppose the main field magnetic flux. In brush-type permanent magnet motors, it is often necessary to provide some type of protection against demagnetization, such as the use of a soft iron pole shoe between the armature and the magnet. The high permeability of the pole shoe provides a low reluctance shunt path for the armature reaction flux around the permanent magnet, thus protecting it from being demagnetized. However, in an ECM having notches in the face of the salient pole to reduce cogging, the continuity of any magnetic flux shunt path around the permanent magnet is interrupted by the notches.
Furthermore, it has been found that the introduction of notches on the face of the salient pole also degrades the overall performance of the motor. When magnetic material is taken away to form the notch, the effect is the same as that of increasing the air gap between the face of the permanent magnet rotor and the salient pole. The presence of a larger air gap reduces the magnetic flux density, and thus affects the performance of the motor. This performance degradation can be manifested in terms of a lower efficiency or a loss of torque.
A need, therefore, exists for a permanent magnet motor having an improved stator configuration to reduce cogging, while at the same time maintaining a high efficiency and a high demagnetization margin.